During semiconductor fabrication, a variety of photolithographic processes are performed to apply layers to, or perform implants in, a semiconductor wafer. A photosensitive photoresist is applied to the wafer, and patterned using a photomask to form a hard mask for a subsequent deposition or etching process. The cost of photoresist is a significant material cost in semiconductor fabrication.
Some polyimide compositions are photosensitive, and can serve as a photoresist. Both “positive” and “negative” types of polyimide photoresist are used. Compared to other types of photoresist, polyimide has a much higher viscosity at its dispensing temperature. For example, some photoresists have a viscosity as low as 10 centipoise, and flow easily. Polyimide can have a viscosity of about 3000 centipoise, and does not spread as readily during dispensing.
If a high spinner rotation speed is used to spread polyimide over the surface of the wafer, a greater portion of the polyimide is spun off of the wafer by centrifugal force and wasted.